Flexure pivots with a virtual pivot can substantially improve timepiece resonators. The simplest are crossed-strip pivots, formed of two straight, generally perpendicular, strips that intersect. These two strips may be either three-dimensional in two different planes, or two-dimensional in the same plane, in which case they are as welded at their crossing point.
It is possible to optimise a three-dimensional crossed-strip pivot for an oscillator, to make it isochronous, with a rate that is independent of its orientation in the field of gravity, in two particular ways (independently, or combined):
selecting the position of the crossing of the strips with respect to their clamping point to achieve a rate independent of position;
selecting the angle between the strips to be isochronous, and achieve a rate independent of amplitude.
Such three-dimensional systems. or systems at least on several levels, are known from EP Patent 2911012 in the name of CSEM, which discloses a rotary oscillator for timepieces comprising a support element to allow assembly of the oscillator in a timepiece, a balance wheel, a plurality of flexible strips connecting the support element to the balance wheel and capable of exerting a return torque on the balance wheel, and a felloe mounted integrally with the balance wheel. The plurality of flexible strips comprises at least two flexible strips including a first strip disposed in a first plane perpendicular to the plane of the oscillator, and a second strip disposed in a second plane perpendicular to the plane of the oscillator and secant with the first plane. The geometric axis of the oscillation of the oscillator is defined by the intersection of the first plane and the second plane, this geometric axis of oscillation crossing the first and second strips at ⅞ths of their respective length. This arrangement is known from the work by Wittrick starting from 1948 on flexure pivots.
EP Patent 1013949 in the name of SYSMELEC discloses a pivot formed of a fixed base and a movable member connected by a flexible structure, with an intermediate element connected to the base and the movable element, respectively, by two pairs of flexible arms. Each of the arms includes a joint at each end, formed by semi-circular recesses creating flexible areas. The pivot further comprises a kinematic control circuit connecting the base and the movable element and the intermediate element, such that the angular motion of the intermediate element corresponds to that of the movable element.
These known solutions have drawbacks however:
a pivot with three-dimensional crossed strips cannot be etched in a single two dimensional etch, which complicates manufacture;
a two-dimensional crossed-strip pivot, with strips welded at the crossing point, is four times stiffer than the equivalent three-dimensional pivot, its permitted travel is four times less than the three-dimensional pivot, and it cannot achieve a rate independent of both position and amplitude.